Integrated on-chip polarizer

ABSTRACT

A low loss high extinction ratio on-chip polarizer is disclosed. The polarizer includes an input waveguide taper having an outer waveguiding region that widens in the direction of light propagation along at least a portion of the taper length, and a core waveguiding region that narrows in the direction of light propagation along at least a portion of the taper length, so as to selectively squeeze out light of undesired modes into the outer regions while preserving light of a desired mode in the waveguide core. An output filter section is provided to prevent light from reentering the output waveguide after being squeezed out. An integrated light absorber/deflector may be coupled to the outer waveguiding regions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/126,864 filed Sep. 10, 2018, now allowed, which is acontinuation-in-part of U.S. patent application Ser. No. 15/725,450,filed Oct. 5, 2017, now U.S. Pat. No. 10,261,254, which is acontinuation of U.S. patent application Ser. No. 14/989,436, filed Jan.6, 2017, now U.S. Pat. No. 9,810,840, each of which is herebyincorporated by reference herein in its entirety.

FIELD OF THE INVENTION

The invention generally relates to photonic integrated circuits, andmore particularly relates to an integrated optical waveguide polarizerand/or mode filter for photonic integrated circuits and related methods.

BACKGROUND OF THE INVENTION

Optical devices that are used in photonic integrated circuits (PICs),such as but not exclusively those formed in a silicon layer of asilicon-on-isolator (SOI) chip, are typically of planar geometry andoperate best with light in a specific polarization state, and preferablyin a fundamental mode. However, higher order waveguide modes could beexcited in PIC waveguides due to various waveguide imperfections such assidewall roughness, transitions between multimode and single moderegions, at fiber coupling, and other interactions of the optical signalwith the geometry of the devices.

Generally, planar waveguides of the type conventionally used in a PICcan support modes of two orthogonal transverse polarization states,termed TE and TM, with the lower-order modes typically being betterconfined within the waveguide and characterized by a greater effectiveindex than higher-order modes of the same polarization. The lowest-ordermodes of each polarization state are commonly referred to as thefundamental modes and denoted as TE0 and TM0, respectively.

In order to optimize PIC performance and reduce noise it is generallydesired that light propagating in the PIC belongs to a fundamental modeof a particular polarization, most commonly TE0. While suppressinghigher-order TE and TM modes may be affected by using sufficientlynarrow waveguides that are often referred to as single-mode, suchwaveguides typically support the fundamental mode of both the TE and TMpolarization. Hence, additional efforts may be needed to discriminatebetween the TE and TM light in a PIC and to suppress one of them.

One possible approach to solving this problem is to use a directionalcoupler or a multi-mode interference (MMI) coupler as a polarizationsplitter to split TE and TM modes in space and couple them intodifferent waveguides. Directional couplers are however sensitive tovariations in wavelength, which makes it difficult to achieve high TM/TEextinction ratio across a suitably wide wavelength range, for exampleacross the entire C band. Another drawback of directional couplers istheir low tolerance to fabrication inaccuracies. A drawback of using anMIMI coupler for splitting the TE and TM polarizations relates to thecoupler length, which may have to be relatively big since the differencebetween effective indices of the TE0 and TM0 modes is typically small.Furthermore, an MMI coupler capable of splitting off the TM polarizationmay have a relatively high insertion loss for the TE mode. Proposedwaveguide polarizers based on asymmetrical Mach-Zehnder Interferometer(MZI) and adiabatic couplers suffer from similar drawbacks, including abig device length and a relatively high insertion loss for the TE mode.

Accordingly, it may be understood that there may be significant problemsand shortcomings associated with current solutions and technologies forproviding a required level of suppression of light of undesiredpolarization and/or modes in photonic integrated circuits.

SUMMARY OF THE INVENTION

Accordingly, one aspect of the present disclosure relates to a low-loss,high extinction ratio optical waveguide polarizer that may be integratedinto a PIC and that may discriminate between fundamental polarizationmodes that can propagate in the PIC.

One aspect of the present disclosure provides a waveguide polarizer,comprising: an input optical waveguide capable of supporting a firstmode and a second mode; an output optical waveguide for outputting thefirst mode; and a mode-selective expander (MSE), extending between theinput and output optical waveguides. The MSE comprising: a modeseparating section for separating the first mode from the second mode;and an output filter section for preventing the second mode fromentering the output optical waveguide.

In a preferred embodiment, the mode separating section comprises: aridge waveguiding region disposed to receive light of the first andsecond modes from the input optical waveguide, and an outer slabwaveguiding region disposed alongside the core waveguiding region inoptical communication therewith. Accordingly, the ridge waveguidingregion and the outer waveguiding region are configured to expand thelight of the second mode from the ridge waveguiding region into theouter waveguiding region, and to propagate the first mode along theridge waveguiding region for coupling into the output optical waveguide,so that the outer waveguiding region remains substantially absent of thefirst mode.

In a preferred embodiment, the output filter section comprises a pair ofangled surfaces, each at an acute angle to the output waveguide onopposite sides thereof, both facing substantially rearwardly towards themode separating section for reflecting light in the slab waveguidingregion away from the output waveguide on opposite sides thereof.

In yet another aspect of the present disclosure, the output opticalfilter comprises a pair of wedge-shaped sections in the outer slabwaveguiding region on opposite sides of the output waveguide forming theangled surfaces; wherein the wedge-shaped sections, each comprises amaterial with a lower index of refraction than the slab waveguidingregion.

The foregoing and other objects, aspects, features, and advantages ofthe invention will become more apparent from the following descriptionand from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments disclosed herein will be described in greater detail withreference to the accompanying drawings, which may be not to scale and inwhich like elements are indicated with like reference numerals, andwherein:

FIG. 1 is a schematic plan-view diagram of an integrated waveguidepolarizer incorporating a bi-level mode-selective expander waveguide;

FIG. 2 is a schematic diagram of a partial cross-section of an inputwaveguide of the integrated waveguide polarizer of FIG. 1 in oneembodiment thereof;

FIG. 3 is a schematic diagram of a partial cross-section of an inputwaveguide taper section of the waveguide polarizer of FIG. 1 in oneembodiment thereof;

FIG. 4 is a schematic diagram of a partial cross-section of a modesqueezing waveguide section of the waveguide polarizer of FIG. 1 in oneembodiment thereof;

FIG. 5 is a schematic plan-view diagram of an embodiment of thepolarizer of FIG. 1 incorporating an optical absorber/deflector in theouter waveguiding regions thereof;

FIG. 6 is a schematic diagram of a partial cross-section along a C-Cline of an embodiment of the waveguide polarizer of FIG. 5 includingdoped outer waveguiding regions;

FIG. 7 is a schematic diagram of a partial cross-section along a C-Cline of an embodiment of the waveguide polarizer of FIG. 5 includingoptical absorber disposed over outer waveguiding regions;

FIG. 8 is a schematic diagram of a partial cross-section along a C-Cline of an embodiment of the waveguide polarizer of FIG. 5 includingoptical absorbing or waveguiding layers disposed over the upper claddingin the outer waveguiding regions;

FIG. 9 is a schematic plan-view diagram of an embodiment of thepolarizer of FIG. 5 incorporating a diffraction grating in the outerwaveguiding regions thereof;

FIG. 10A is a graph showing a simulated two-dimensional (2D)distribution of the electric field (E-field) of a TE0 mode in an inputoptical waveguide of an example embodiment of the polarizer of FIG. 1,at cross-section (a) in FIG. 10D;

FIG. 10B is a graph showing a simulated 2D distribution of the E-fieldof the TE0 mode in an input rib waveguide taper of the exampleembodiment of the polarizer of FIG. 1, at cross-section (b) in FIG. 10D;

FIG. 10C is a graph showing a simulated 2D distribution of the E-fieldof the TE0 mode in the middle of the squeezing waveguide of the exampleembodiment of the polarizer of FIG. 1, at cross-section (c) in FIG. 10D;

FIG. 10D is a schematic representation of the polarizer of FIG. 1showing the locations (a), (b), and (c) at which the 2D E-fielddistributions illustrated in FIGS. 10A-C were computed;

FIG. 11A is a graph showing a simulated 2D distribution of the E-fieldof light of an input TM0 mode in an input optical waveguide of theexample embodiment of the polarizer of FIG. 1, at polarizercross-section (a) in FIG. 11D;

FIG. 11B is a graph showing a simulated 2D distribution of the E-fieldof the light of the input TM0 mode in the input ridge-to-rib waveguidetaper of the example embodiment of the polarizer of FIG. 1, at polarizercross-section (b) in FIG. 11D;

FIG. 11C is a graph showing a simulated 2D distribution of the E-fieldof the light of the input TM0 mode in the rib waveguide taper of theexample embodiment of the polarizer of FIG. 1, at polarizercross-section (c) in FIG. 11D;

FIG. 11D is a schematic representation of the polarizer of FIG. 1showing the locations (a), (b), and (c) at which the 2D E-fielddistributions illustrated in FIGS. 11A-11C were computed;

FIG. 12A is a 2D plot illustrating the evolution of an in-plane E-fieldprofile along the length of the example polarizer of FIG. 5 with dopedouter waveguiding regions, for input light in the TE0 mode;

FIG. 12B is a 2D plot illustrating the evolution of an in-plane E-fieldprofile along the length of the example polarizer of FIG. 5 with dopedouter waveguiding regions, for input light in the TM0 mode;

FIG. 13A is a graph showing simulation results for the wavelengthdependence of an insertion loss for TE0 light of the example polarizerof FIG. 5 with doped outer waveguiding regions;

FIG. 13B is a graph showing simulation results for the wavelengthdependence of the polarization extinction ratio at the output of theexample polarizer of FIG. 5 with doped outer waveguiding regions;

FIG. 14 is a schematic plan-view diagram of an embodiment of thepolarizer of FIG. 1 incorporating an optical shielding;

FIG. 15 is a schematic plan-view diagram of a photonic integratedcircuit with an optical waveguide interconnect incorporating integratedwaveguide polarizers or mode filters for suppressing signal noise due topolarization mode cross-coupling;

FIG. 16 is a schematic plan-view of diagram of another embodiment; and

FIG. 17 is a schematic plan-view diagram of an embodiment of thepolarizer of FIG. 16 incorporating an optical shielding.

DETAILED DESCRIPTION

In the following description, for purposes of explanation and notlimitation, specific details are set forth, such as particular opticalcircuits, circuit components, techniques, etc. in order to provide athorough understanding of the present invention. However, it will beapparent to one skilled in the art that the present invention may bepracticed in other embodiments that depart from these specific details.In other instances, detailed descriptions of well-known methods,devices, and circuits are omitted so as not to obscure the descriptionof the present invention. All statements herein reciting principles,aspects, and embodiments of the invention, as well as specific examplesthereof, are intended to encompass both structural and functionalequivalents thereof. Additionally, it is intended that such equivalentsinclude both currently known equivalents as well as equivalentsdeveloped in the future, i.e., any elements developed that perform thesame function, regardless of structure.

Furthermore, the following abbreviations and acronyms may be used in thepresent document:

CMOS Complementary Metal-Oxide-Semiconductor

GaAs Gallium Arsenide

InP Indium Phosphide

LiNO₃ Lithium Niobate

MFD Mode Field Diameter

MPW Multi Project Wafer

PIC Photonic Integrated Circuits

PSO Particle Swarm Optimization

SOI Silicon on Insulator

TE Transverse Electrical

TM Transverse Magnetic

Note that as used herein, the terms “first”, “second” and so forth arenot intended to imply sequential ordering, but rather are intended todistinguish one element from another, unless explicitly stated.Similarly, sequential ordering of method steps does not imply asequential order of their execution, unless explicitly stated. The word‘using’, when used in a description of a method or process performed byan optical device such as a polarizer or a waveguide, is to beunderstood as referring to an action performed by the optical deviceitself or by a component thereof rather than by an external agent. Theterm ‘TE mode’ refers to a waveguide mode with the direction of theelectric field vector transverse, i.e. orthogonal, to the direction oflight propagation. The term ‘TM mode’ refers to a waveguide mode withthe direction of the magnetic field vector transverse, i.e. orthogonal,to the direction of light propagation. In a planar waveguide, theelectric field of a TE mode may lie primarily in the plane of thewaveguide, that is in the plane of a chip supporting the waveguide,while the electric field of a TM mode may lie primarily in a planenormal to the plane of the waveguide, or normal to the plane of a chipsupporting the waveguide. Accordingly, TE and TM modes may also bereferred as polarization modes. TE and TM modes of an n-th order aredenoted as TEn and TMn, respectively, with n=0 designating a fundamentalmode, so that notations TE0 and TM0 designate the fundamental TE and TMmodes, respectively. The term ‘higher-order mode’ may refer to anynon-fundamental TEn or TMn mode of the order n=1, 2, 3, . . . greaterthan zero, unless explicitly stated otherwise. An optical device thatsuppresses light of a selected mode or group of modes to a greaterdegree than light of another mode or group of modes may be referred toherein as a mode filter or mode throttler. An optical device thatsuppresses light of one polarization, for example TM or TE, to a greaterdegree than light of an orthogonal polarization, for example TE or TM,is referred to herein as an optical polarizer. It will be appreciatedthat a waveguide polarizer may also be referred to as a mode filter.

One aspect of the present disclosure relates to mode filters orpolarizers that strip incoming optical signal from substantially allmodes that may be present therein except for one desired mode, and letthe desired mode to propagate further along an optical pathsubstantially unaffected. Some embodiments may relate to mode filters orpolarizers that strip incoming optical signal from one or more undesiredmodes that may be present therein, and let the desired mode to propagatefurther along an optical path substantially unaffected. The desired modemay be for example a fundamental TE or TM mode, but may also be a higherorder TE or TM mode. Such devices are useful in many applications wherea good optical beam quality is of importance. Integrated waveguidepolarizers or mode filters that may be incorporated on a chip may beparticularly useful in photonic integrated circuits (PIC) as means toeliminate or suppress noise due to mode crosstalk and/or polarizationcrosstalk that can otherwise be generated as an optical signal passesthrough a conventional PIC. An integrated on-chip waveguide polarizerincorporated in a PIC enables to decrease polarization crosstalk in thePIC and increase signal to noise ratio for optical signals in the PIC.

One or more example embodiments described herein relate to an integratedwaveguide polarizer with ultra-low loss for light of a desiredpolarization mode, for example on the order of 0.05 dB or less, and ahigh increase in polarization extinction ratio (ER), for example on theorder of 20 dB or greater, which may be advantageously used in a PICwhere eliminating a TM0 mode and/or higher-order TEn modes whileavoiding a loss to a TE0 mode is desired. In these embodiments, theintegrated polarizer expands light that enters the polarizer in theunwanted mode into an outer waveguiding region or regions where it isthen selectively eliminated, for example absorbed or diverted away fromthe optical path. At the same time, the desired mode, such as thefundamental TE0 mode, remains substantially confined within a corewaveguiding region of the polarizer and is guided by it to an output endof the polarizer substantially without loss or with only a minimal lossin power. Generally, the waveguide polarizer described herein may beconfigured to pass through a desired mode while selectively suppressingany undesired mode having a lower effective index in an input waveguideand/or an output waveguide than the desired mode. The desired mode mayalso be referred to herein as the first mode and may be generallydenoted M1, while the unwanted mode may also be referred to herein as asecond mode and may be generally denoted as M2.

The first and second modes M1 and M2 may be, for example, waveguidemodes of an input waveguide of the polarizer, with the first mode M1typically characterized by a greater effective index in the waveguide.The example integrated waveguide polarizer, embodiments of which aredescribe hereinbelow, may be configured to pass the first mode M1therethrough substantially without loss, while substantially blockingthe second mode M2; accordingly, the first mode M1 may be referred to asthe desired mode, while the second mode M2—as the undesired mode. Itwill be appreciated that whether a TM mode or a TE mode has the greatesteffective index in a waveguide, and therefore is best confined in thewaveguide core, may depend on the waveguide geometry. In one embodiment,the first mode M1 may be a fundamental TE or TM mode, while the secondmode M2 may be a higher-order mode having a lower effective index thanthe first mode M1. In one embodiment, the first mode M1 may be afundamental TM mode, while the second mode M2 may be a fundamental TEmode, i.e. TE0, or a higher-order TM or TE mode. In one embodiment, thefirst mode M1 may be a fundamental TE mode, i.e. TE0, while the secondmode M2 may be a fundamental TM mode, i.e. TM0, or a higher-order TM orTE mode. In a representative embodiment, the first mode M1 may be theTE0 mode, the second mode M2 may be the TM0 mode.

With reference to FIG. 1, there is illustrated a schematic plan view ofan integrated optical polarizer 100 that is configured to strip inputlight 101 of an undesired second mode M2 while allowing light of adesired first mode M1 to pass through the polarizer substantiallywithout loss or with only a small loss in power. The polarizer 100 maybe formed of, or include, a mode-selective expander (MSE) waveguide 110of a variable width or widths that may be configured to selectivelyattenuate, or squeeze out, the light that enters the polarizer 100 inthe second mode M2 while allowing the light that enters the polarizer100 in the first mode M1 to propagate to an output 125. The MSEwaveguide 110, which may also be referred to herein as the MSE 110, maybe in the form of a bi-level, or generally multi-level, waveguide thathas a core waveguiding region 122 of a first width We that may vary inthe direction of light propagation, as indicated by an arrow 131, and anouter waveguiding region 123 adjoining the core waveguiding region 122alongside thereof, the outer waveguiding region 123 being of a secondwidth Ws that may also vary in the direction of light propagation. Thecore waveguiding region 122, which may also be referred to herein as thewaveguide core 122 or simply as the core 122, is disposed to receivelight 101 of the first and second modes from an input optical waveguide105. The core and/or outer waveguiding regions 122, 123 are configuredto preferentially expand the light of the second mode M2, which may havea lower effective index n_(eff) than the first mode M1, from the corewaveguiding region 122 into the outer waveguiding region 123, asschematically indicated by a curved arrow 132, and to propagate thefirst mode M1 along the core waveguiding region 122 for coupling into anoutput waveguide 125, as schematically indicated at 131, so that theouter waveguiding region 123 remains substantially absent of the firstmode. The MSE 110 may be disposed on a PIC chip 150 as a part of a PIC.The first mode M1 may be, for example the TE0 mode of the inputwaveguide 105, while the second mode M2 may be a TM0 mode or ahigher-order TMn or TEn mode of the input waveguide 105 having a lowereffective index than the first mode. The outer waveguiding region 123 isgenerally greater in width than the input optical waveguide 105, whilethe core waveguiding region 122 of the MSE 110 may be smaller in widththan the input optical waveguide 105 along at least a portion of thelength of the MSE 110.

The MSE waveguide 110 may include an input taper section 114 wherein theouter waveguiding region 123 gradually widens in the direction of lightpropagation and the core waveguiding region 122 gradually narrows in thedirection of light propagation. The input taper section 114 is opticallyfollowed by a mode squeezing waveguide section 116 wherein the core 122remains narrower than in the input waveguide 105 along a waveguidelength L_(sq), with the core width Wc1 that is smaller than the corewidth Wc in the input waveguide 105. In one embodiment, the modesqueezing waveguide 116 may be followed by an output taper section 118,which may be configured to provide an adiabatic transition to an outputwaveguide 125 for the desired mode M1. In embodiments wherein the inputwaveguide 105 and the output waveguide 125 are of substantially samegeometry and material structure, the output taper section 118 may mirrorthe input taper section 114. Embodiments may be envisioned wherein theoutput taper section 118 may be absent, for example when the outputlight 131 from the polarizer is to be coupled to a free-space optics orto a waveguide of a same core width as the width Wc1 of the waveguidecore 122 in the mode squeezing waveguide 116.

FIG. 1 illustrates an example embodiment wherein the outer waveguidingregion 123 gradually widens along a first length portion 112 of theinput taper section 114, while the core waveguiding region 122 graduallynarrows along a second length portion 113 of the input taper section114, which follows the first length portion 112 in the direction oflight propagation. In other embodiments the widening of the outerwaveguiding section 123 may occur in parallel with the narrowing of thecore 122, along a same length portion of the polarizer.

Referring now also to FIGS. 2, 3, and 4 while continuing to refer toFIG. 1, in one embodiment the input waveguide 105 may be a ridgewaveguide of height h1 and width Wc, while the MSE 110 may be in theform of a bi-level strip-loaded waveguide, which may also be referred toas the bi-level rib waveguide 110, wherein the core waveguiding region122 in the form of a ridge or rib of height h1 is flanked on one or bothsides by a slab waveguide of height h2 <h1 defining the outerwaveguiding region or regions 123. The outer waveguiding regions 123 mayalso be referred to herein as the slab waveguide 123. The first lengthportion 112 of the MSE 110, wherein the slab waveguide 123 widens whilethe width of the core waveguide 122 may remain the same, may also bereferred to in this embodiment as the ridge-to-rib waveguide taper 112,while the second length portion 113 wherein the core waveguide 122narrows may also be referred to as the rib waveguide taper 113. FIG. 2shows a cross-sectional view of the input ridge waveguide 105 takenalong the A-A line indicated in FIG. 1. FIGS. 3 and 4 showcross-sections of the bi-level strip-loaded waveguide 110 in the inputtaper section 114 and in the mode squeezing waveguide 116, which aretaken along the lines B-B and C-C indicated in FIG. 1, respectively.FIG.3 shows the cross-section of the ridge-to-rib waveguide taper 112,wherein the width Wc of the core 122 remains constant and matching thewidth of the input ridge waveguide 105, while the width Ws of the outerwaveguiding portion 123 gradually widens. In the rib waveguide taper 113the width Wc of the waveguiding core 122 gradually decreases to thesmaller width Wc1 of the waveguide core in the squeezing waveguide 116,which cross-section is illustrated in FIG. 4.

In other embodiments, the input waveguide 105 may also be a bi-level ribwaveguide, for example with an outer slab waveguide of a differentheight h3 h2, with the first length portion 112 of the input taper 114providing an adiabatic transition and mode matching between the inputwaveguide 105 and the rib waveguide taper 113.

Continuing to refer to FIGS. 1-4, the input ridge waveguide 105 and thebi-level rib waveguide 110 may be disposed in or upon the PIC chip 150and formed, for example, by selectively etching to a desired depth asemiconductor or dielectric layer 133 that is disposed over a base layer142. The base layer 142 has a lower index of refraction than thematerial of the core and outer waveguide regions 122, 123 of the ribwaveguide 110, and serves as a lower cladding layer of the ridge or ribwaveguide. The bi-level rib waveguide 110 may also be formed by atwo-step selective deposition process of same or different materialsthat forms top and bottom layers of different width. In one embodiment,a top cladding layer 143 may be deposited over the rib waveguide 110. Inanother embodiment, the top layer 143 may be absent, with the uppercladding provided by air or another gas. In one embodiment the PIC chip150 may be a silicon-on-insulator (SOI) chip 150, the layer 133 in whichthe core and outer waveguiding regions 122, 123 of the rib waveguide 110is formed may be a silicon layer of the SOI chip 150, and the base layer142 may be a layer of silicon dioxide (SiO2) disposed over a siliconsubstrate 141 of the SOI chip 150. The top cladding layer 143, whenpresent, may be for example a layer of silicon dioxide, silicon nitride(Si3N4), polymer, or of any other suitable non-conducting material of alower index of refraction than that of the waveguiding regions 122, 123.It will be appreciated that other material systems may also be used toform the bi-level rib waveguide 110.

In one embodiment, the first mode M1 may be the fundamental TE0 mode ofthe input ridge waveguide 105 having the highest effective index neffand therefore the largest optical confinement in the waveguide core,with the TM0 mode and the higher-order TE modes TEn having lowereffective indices than that of the TE0 mode. The narrowing down of thecore waveguiding region 122 along the second length portion 113 of theinput taper 114 squeezes the TM0 mode and the higher-order TEn modes outof the waveguide core 122 and into the outer waveguiding regions 123 toa significantly greater extent than the higher-effective index firstmode TE0. The widths of the slab waveguide Ws and of the core waveguideWc1 in the mode squeezing section 116 may be selected so that opticalpower of the first mode TE0 is confined substantially within thewaveguide core 122, while the TM0 and TEn modes may have most of themode power spread in the outer waveguiding regions 123 where the firstmode TE0 is substantially absent or negligibly small, for example, nomore than a few percent or tenth of a percent in power. Furthermore, thegeometry of the input waveguide taper 114 may be configured so thatlight that enters the MSE 110 in a TM0 mode may be at least partiallycoupled into a higher-order TEn mode, such as TE1 and TE3, within alength of the ridge-to-rib waveguide taper 112, and is then squeezed outof the waveguide core 122 into the side region 123 as the waveguide core122 becomes narrower in the rib waveguide taper 113. The TM→TE modeconversion of this type may be effected in the bi-level rib waveguidetaper 110 at a specific width Ws=Wsc of the outer regions 123 due to its“vertically asymmetry,” i.e. an asymmetry in the direction normal to thesurface of the PIC wafer or layer upon which the waveguide 110 isformed, which is the z-axis direction in the example coordinate systemillustrated in FIGS. 1-4. The TM0→TEn conversion in the ridge-to-ribtaper 112 may facilitate the suppression of light that enters thepolarizer in the TM0 mode, for example, in embodiments wherein theeffective index of the TM0 mode is higher than that of the TE1 mode inthe narrow-core squeezing waveguide 116.

Generally, the geometry of the input waveguide taper 114, which may bedefined by the height parameters of the rib waveguide h1 and h2, thewidth Ws1 and Wc1 of the slab waveguide 123 and of the waveguide core122 at the output end of the input taper 114, respectively, may beselected so that light 101 that enters the MSE 110 in the first modehaving the greatest effective index in the input waveguide 105, such asfor example the TE0 mode, remains largely confined within the waveguidecore 122 at the output of the input taper section 114 and in the modesqueezing waveguide 116, while light 101 that enters the MSE 110 in thesecond mode having the lower effective index in the input waveguide 105,such as for example the TM0 mode or a TEn mode, largely loses itsconfinement within the waveguide core 122 by the time it reaches themode squeezing waveguide 116. It may be preferable that the input tapersection 114 is an adiabatic taper of a sufficiently long length, so thatthe change of its core width Wc and of the width of the outerwaveguiding sections Ws happens smoothly over a length that issufficient to prevent back reflections and to allow light that entersthe input taper section 114 in the second mode, for example TM0, tocouple into the higher-order modes and/or modes of the slab waveguide123.

Generally, the width and height parameters Wc, Wc1, Ws, h1, h2 of therib waveguide 110 of the polarizer 100 in various sections may depend onthe core and cladding materials of the waveguide and the targetwavelength range of operation, and one skilled in the art will be ableto determine suitable values using commercially available software forwaveguide simulations and experimental verification. By way of examplefor the polarizer 100 that is formed in a h1=220 nm thick silicon layerof a SOI chip, the input waveguide width Wc may be in the range of 0.4to 0.6 μm, the slab waveguide thickness h2 may be in the 50 to 160 nmrange, and the core waveguide width Wc1 of the squeezing waveguide 116may be down to 0.18 to 0.24 μm. The lengths of the ridge-to-rib taper112 L1 and of the rib waveguide taper 113 L2 may be selected to providean adiabatic transition between the input ridge waveguide 105 and thesqueezing rib waveguide 116; for example, each of L1 and L2 may be about10 μm or greater. The length of the squeezing rib waveguide 116 L3 maybe selected to provide a desired level of suppression of the unwanted TMand TEn modes, and may also be for example about 10 μm or greater. Thewidth Ws of the outer regions of the rib waveguide in a middle portionof the structure 100 may be for example 5 μm or greater.

Note that although the narrowing of the waveguide core 122 in the ribwaveguide taper 113 and the widening of the slab waveguide 123 in theridge-to-rib taper 112 is shown in FIG. 1 to be linear with devicelength in the direction of light propagation, it is by example only andin other embodiments the narrowing of the core waveguide 122 and/or thewidening of the slab waveguide 123 may be non-linear with the distancealong the direction of light propagation, and the input taper section114 may be a multi-segmented and/or smooth taper, which exact shape inthe plane (x,y) of the chip 150 may be determined by optimization.

In one embodiment, light 132 of the unwanted second mode, such as theTM0 and/or TEn, that is squeezed out of the waveguide core 122 in thesqueezing waveguide 116, may be scattered away from the waveguide core122 so that only at most a small portion of it is coupled back into thewaveguide core by the outer taper 118; in some embodiments, suchscattering may be sufficient to provide a desired level of suppressionof the undesired modes at the output of the polarizer 100.

Referring to FIG. 5, in some embodiments the MSE 110 may include a lightabsorber/deflector 160 that is configured to selectively absorb ordeflect light propagating in the outer waveguiding region 123 thereof,and in particular in the outer waveguiding regions of the mode squeezingwaveguide 116. Preferably, the light absorber/deflector 160 is locatedin a region of the MSE 110 which is substantially absent of the firstmode TE0, at a distance d from the waveguide core 122 that exceeds thepenetration depth of the first mode TE0 into the slab waveguide 123within the mode squeezing waveguide 116; accordingly, the light 131 ofthe first mode TE0 can propagate in the waveguide core 122 substantiallywithout attenuation by the light absorber/deflector 160. The lightabsorber/deflector 160 may be, for example, in the form of a metal layercoupled to the outer waveguiding region 123 of the MSE 110, a layer oflight-absorbing semiconductor material coupled to the outer waveguidingregion 123 of the MSE 110, or a doped region of a semiconductor layerforming the outer waveguiding region of the MSE 110. In otherembodiments, the light absorber/deflector 160 may be an element thatdeflects light propagating in the slab waveguide 123 away from thewaveguiding core 122, such as for example an optical grating or a lightreflecting or deflecting grove that may be formed in the slab waveguide123.

With reference to FIG. 6, in one embodiment wherein the mode squeezingwaveguide 116 is defined in a layer of a semiconductor material, such asfor example the silicon layer of a SOI chip, the lightabsorber/deflector 160 may be in the form of a suitably doped region orregions 162 of the slab waveguide 123. A suitably high level of dopingof the slab waveguide 123 may drastically increase the concentration offree carriers, i.e. electrons or holes, in the doped regions 162,thereby making it electrically conductive. The doped regions 162 formedin the portion of the slab waveguide 123 where the undesired TM or TEnlight 132 penetrates will absorb the undesired light due to the freecarrier absorption, thereby increasing the extinction ratio of theundesired light at the output of the polarizer. It will be appreciatedthat the doped regions 162 may be either n-doped or p-doped, and may beformed using well-developed in the art technologies, such as for exampleby selective diffusion or ion implantation of suitable dopants. By wayof example the slab waveguide 123 may be made of silicon, and the dopedregion 162 may be a p++ region formed by selectively doping the siliconslab waveguide 123 with boron (B) to a doping concentration in the rangeof about 5·10¹⁸-10²⁰ cm⁻³. The doped region 162 may be a n++ regionformed by selectively doping the silicon slab waveguide 123 withphosphorus (Ph) to a doping concentration in the range of about5·10¹⁸-10²⁰ cm⁻³. It will be appreciated that other dopant materialsand/or other doping levels may also be used.

With reference to FIG. 7, in another embodiment the lightabsorber/deflector 160 may be in the form of a conducting layer orlayers 164 that may be disposed upon the slab waveguide 123 at thesuitable distance d from the waveguide core 122. The conducting layer orlayers 164 may be, for example, a layer of a suitable metal, such as forexample, Cu and/or Al and/or their alloy, or a layer of a semiconductormaterial that is not transparent at the wavelength λ, to the input light101. For example, in embodiments wherein the polarizer 100 is configuredto operate in the 1.3-1.55 μm wavelength range, the slab waveguide 123may be defined in a layer of undoped silicon, and the conducting layeror layers 164 may be a germanium (Ge) layer.

With reference to FIG. 8, in another embodiment the lightabsorber/deflector 160 may be in the form of an auxiliary layer 166 thatmay be disposed upon the top cladding layer 143 over the slab waveguide123 at a small distance therefrom so as to be optically coupled to theslab waveguide, but at a suitably large distance from the waveguide core122 to prevent out-coupling thereto of the light 131 of the first, ordesired, mode, e.g. the TE0 light. The auxiliary layer or layers 166 maybe for example made of a light-absorbing material such as a suitablemetal or a light-absorbing semiconductor, for example germanium. Theauxiliary layer or layers 166 may also be a waveguiding layer whereinlight propagating in the slab waveguide 123 may couple into, and whichmay be configured to guide that light away from the core waveguide 122and from the polarizer's output. By way of example, the auxiliary layer166 may be a layer of a dielectric or semiconductor material that has agreater index of refraction than that of the top cladding layer 143,such as for example Si, SiN, or SiON, among others.

Referring now to FIG. 9, in one embodiment the light absorber/deflector160 may be in the form of a diffraction grating 230 that may be formed,for example by etching, in the slab waveguide 123 or in the top claddinglayer 143. The diffraction grating 230 may have a period selected todeflect the undesired light 132 of the undesired second mode or modesaway from the waveguide core 122 and from the polarizer output 125. Forexample, the diffraction grating 230 may be a second-order gratingdesigned to deflect light of the operating wavelength λ, out of the PCIchip 150.

With reference to FIGS. 10A-10D, simulated electrical field (E-field)profiles of propagating light at three different polarizer locationswhen input light 101 enters the polarizer 100 in the fundamental TE0mode are illustrated in FIGS. 10A-10C, with the respective polarizerlocations (a), (b), and (c) indicated in FIG. 10D. Simulations wereperformed for an example polarizer generally as illustrated in FIGS. 1-6formed in the silicon layer of a SOI wafer, with SiOx lower and uppercladdings and doped regions in the outer slab waveguide. The followingdevice parameters were used in the simulations: Si layer thicknessh1=220 nm, the slab waveguide thickness h2=90 nm, the width of the inputridge waveguide 105 Wc=500 nm, the width of the waveguide core in thesqueezing waveguide, i.e. at location (c) in FIG. 10D, Wc1=220 nm. Ascan be clearly seen from the figures, at all three locations along thepolarizer, the power of the TE0 mode is substantially confined in theridge waveguide and does not penetrate into the slab waveguide more thana micron even where the waveguiding core is the narrowest, i.e. atlocation (c) shown in FIG. 10D. Accordingly, a light absorber/deflector160 that is disposed at the distance from the waveguide core of about0.6-0.7 microns or more will not attenuate the TE0 mode to anysubstantial degree.

Turning now to FIGS. 11A-11C, there are illustrated simulated E-fieldmode profiles when input light 101 is in the TM0 mode for the examplepolarizer as described hereinabove with reference to FIGS. 10A-10C, atpolarizer locations (a), (b), and (c) indicated in FIG. 11D. As can beseen in FIGS. 11B and 11C, input light that enters the polarizer in theTM0 mode is largely converted to a higher-order mode or modes in theinput waveguide taper where the slab waveguide widens and the waveguidecore narrows. The coupling to higher-order modes of the slab waveguidestarts in the ridge-to-rib taper, FIG. 11B and location (b), wherein thelight is seen to spread to form secondary intensity maxima 1-2 micronsaway from the waveguide core. The narrowing of the waveguide core in therib waveguide taper, FIG. 11C and location (d) indicated in FIG. 11D,leads to the input light largely or almost completely converted tohigher-order modes in the slab waveguide, with light intensity spreadingto about +\−4.5 μm away from the core waveguide axis, which correspondsto y=0 in FIGS. 10A-10C and 11A-11C.

As can be seen from FIGS. 10A-10C and 11A-11C, the E-field, andtherefore the optical power, of the TE0 mode is substantially confinedin the core waveguide along the whole length of the polarizer and doesnot penetrate into the slab waveguide more than by about half of amicron even where the waveguiding core is the narrowest, i.e. atlocation (c) in FIG. 10D. Accordingly, a light absorber/deflector 160that is disposed at a distance from the waveguide core center of about0.6-0.7 microns or more will not attenuate the TE0 mode to anysubstantial degree. However, the intensity of light that enters thepolarizer in the TM0 mode is spread deep, by 1-4 microns in the shownexample, into in the outer waveguiding regions of the input waveguidetaper due to the narrowing of the waveguide core and the widening of theouter slab waveguide. Accordingly, a light absorber/deflector 160 thatis disposed at the distance from the waveguide core center of about0.7-1 μm with a width of 5-10 μm or more may significantly attenuate theundesired light that enters the polarizer in the TM0 mode.

With reference to FIGS. 12A and 12B, there are illustrated profiles ofthe light intensity along the polarizer length, in the plane of the PICchip, which corresponds to the (x,y) plane in the (x,y,z) coordinatesystem indicated in FIGS. 1 and 5. The profiles were computed for theexample polarizer as described hereinabove with reference to FIGS.10A-10C, with doped regions in the slab waveguide located at a distanced=1.4 microns from the waveguide axis. FIG. 12A shows the evolution ofthe optical E-field profile along the polarizer length for light 101that enters the polarizer in the TE0 mode, while FIG. 12B shows theevolution of the optical E-field profile along the polarizer length forlight 101 that enters the polarizer in the TM0 mode. As can be clearlyseen from FIG. 12A, for TE0 input the optical power is alwaysconcentrated in the core ridge waveguide. Contrary to that, light thatenters the polarizer in the TM0 mode starts to couple to higher-ordermodes in the ridge-to-rib taper, which corresponds to the x-coordinatein the range of 7 to 17 microns in FIGS. 12A and 12B, with its powerlargely squeezed out of the core ridge and into higher-order modes ofthe outer waveguiding regions by the time it reached the middle of thesqueezing waveguide, which corresponds to the x-coordinate in the rangeof 27 to 47 microns in FIGS. 12A and 12B. In the outer waveguidingregions 123 of the MSE 110, the light converted into the higher-ordermodes from the TM0 is attenuated to a negligibly low level due to thefree carrier absorption in the doped regions of the slab waveguide, andsubstantially disappears without reaching the output waveguide (68microns in FIGS. 12A, B).

Turning now to FIGS. 13A and 13B, there are illustrated simulationresults for the wavelength dependence of the polarizer insertion lossfor the TE0 mode (FIG. 13A) and the increase in the polarizationextinction ratio at the polarizer output (FIG. 13B) for the examplepolarizer as described hereinabove with reference to FIGS. 10A-10C withhighly doped outer waveguiding regions. The polarization extinctionratio ER of the polarizer, as illustrated in FIG. 13B, is defined asER=10·log₁₀(P_(TE0)/P_(TM0)), where P_(TE0) and P_(TM0) are the opticalpower of the TE0 mode and of the TM0 mode, respectively, at the outputof the polarizer at equal optical power in the TE0 and TM0 modes at thepolarizer input. As can be seen from the figures, the computed TE0insertion loss stays below 0.2 dB across the C-band of the wavelengthspectrum, i.e. from 1.5 to 1.6 while the insertion loss for the TM0 modeexceed 25 dB across the C-band, corresponding to an increase of theTM/TE extinction ratio at the polarizer output by at least 25 dB. Theextinction ratio performance can be further improved, for example bymaking the slab region of the rib waveguide wider and making the devicelonger.

Turning now to FIG. 14, in one embodiment the integrated waveguidepolarizer 100 may include a shield 188 surrounding the MSE 110 so as toblock or absorb light 132 of the undesired second mode or modes that isspread or scattered into the outer waveguiding regions 123 from leakinginto other optical devices that may be present in the PIC chip 150, andalso to block scattered light from other optical devices from couplinginto the polarizer. The shield 188 may be, for example, in the form of awall of a light absorbing material, such as metal or semiconductor, forexample Germanium. Such a wall may be formed, for example, in a viaformed in the upper cladding layer 143 and, possibly, in the lowercladding layer 142 illustrated in FIGS. 2-4 and 6-7. Alternatively, theshield 188 may be, for example, in the form of a trench formed in thecladding layers. Various embodiments of the shield 188 are described ina co-pending patent application entitled “Shielded Photonic IntegratedCircuit,” which is assigned to the assignee of the present application.In some embodiments, the shield 188 may be used as the lightabsorber/deflector 160 described hereinabove to absorb scattered lightof the undesired second mode or modes and to prevent it from couplinginto the output waveguide 125 of the polarizer 100. In some embodiments,the shield 188 may be disposed only at one side of the MSE 110, forexample when no optical devices or sources of scattered light arepresent at another side thereof.

It will be appreciated that principles and approaches describedhereinabove with reference to the example embodiments, and in particularwith reference to squeezing out from a waveguide core and dumping a TM0mode of input light while allowing light of a TE0 mode to propagate, mayalso be applicable to filtering out other undesired modes whilepreserving a desired mode or modes, by selectively squeezing out anddumping the undesired higher order mode. Accordingly, an aspect of thepresent disclosure provides a method of polarization and/or modefiltering, in a PIC chip comprising an optical waveguide supportingfirst and second modes. The method may comprise: a) receiving lightcomprising first and second modes into a mode-selective expander (MSE)waveguide that comprises a core waveguiding region and an outerwaveguiding region; and, b) using the MSE waveguide to preferentiallyexpand the light of the second mode from the core waveguiding regioninto the outer waveguiding region, and to propagate the first mode alongthe core waveguiding region for coupling into an output waveguide, sothat the outer waveguiding region remains substantially absent of thefirst mode.

In one embodiment of the method, the first mode may be a highesteffective index mode of the input waveguide, and the second mode may beany other mode having a lower effective than that of the first more. Inone embodiment, the first mode may be a TE mode, and the second mode maybe a TM mode. For example, the first mode may be TE0 and the second modemay be TM0. In one embodiment, the first mode may be a TM0 mode and thesecond mode may be a TE0 mode. In one embodiment, the first mode may bea highest effective index mode of the input waveguide, for example theTE0 mode, and the second mode may be a mode having a second highesteffective, for example the TM0 mode. Note that in all such embodimentsthe integrated waveguide polarizer 100 described hereinabove operatessubstantially as a mode filter that is configured to block all modesother than the highest-index one, since the mode squeezing waveguidesection that is configured to eliminate the mode with the second highesteffective index will also eliminate all higher-order TE and TM modesthat pass the input taper section 114. Embodiments wherein the highesteffective index mode is a TM0 mode may also be envisioned, as may bedefined by the waveguide geometry.

Embodiments described hereinabove provide an integrated on-chippolarizer that may be fabricated for example in silicon or other layerof a SOI wafer, in a way that is compatible with a CMOS fabricationprocess. Advantageously, such a polarizer may have ultralow loss for adesired mode, e.g. on the order or less than 0.2 dB for a TE0 mode, anda high extinction ratio, e.g. in excess of 25 dB for the undesired mode,such as e.g. the TM0 mode. Furthermore, optical polarizers or modefilters constructed using principles and methods described herein may beused to filter and or polarized light with wavelengths lying in variouswavelength ranges, including but not limited to the telecommunicationwavelength ranges known as the O-band, E band, S band C band, L band,and U band, which together span from about 1260 nm to about 1675 nm.

Low-loss high-efficiency waveguide polarizers that can be integratedwithin a PIC, including but not limited to those described hereinabove,may be advantageously used to suppress spectral and intensity noise thatis associated with polarization and/or mode cross-coupling in a PIC,such as for example due to mode scattering on waveguide irregularitiesthat may be occurring within long optical waveguide interconnects. Forexample, the integrated waveguide polarizer 100 may be incorporated in aPIC in conjunction with an integrated optical device such as aphotodetector, an optical modulator, an Echelle grating, an MMI coupler,a routing waveguide, an integrated laser source, etc. For example,polarizer 100 may be disposed at an input port of an integratedphotodetector, modulator, MMI coupler, or Echelle grating, so as tofilter out an undesired mode or modes, e.g. the TM0 mode. Similarly, itmay be incorporated at the output of an integrated laser source, MIMIcoupler, or Echelle grating. In some embodiments, one or more waveguidepolarizers may be inserted along the length of a long waveguideinterconnect in a PIC. In some PIC embodiments, long waveguideinterconnects may be required to route light between two opticalwaveguides or ports that are comparatively far from each other in thePIC. In some cases such waveguide interconnects may be made wider andmultimode in order to reduce optical loss. In many cases, suchwaveguides are fed substantially with polarized light, so that most ifnot all of the light may be concentrated in the TE0 mode at the input.However, scattering on waveguide non-idealities may lead to mode andpolarization conversion in such waveguides, when some of the TE0 lightgets scattered into other modes, causing polarization or modecross-coupling that may lead to undesirable noise at the receivingdevice.

Referring to FIG. 15, there is illustrated a PIC 250 including anoptical waveguide interconnect (OWI) 230 that is configured to providean optical connection between a first optical device 210 and a secondoptical device 220. The first optical device 210 may be, for example, anintegrated laser source, a waveguide modulator, a waveguide coupler, orany other optical device configured to transmit an optical signal andwhich may be integrated in a PIC chip. The second optical device 220 maybe, for example, a photodetector, a waveguide modulator, a waveguidecoupler, or any other optical device configured to receive an opticalsignal and which may be integrated in a PIC chip. In one embodiment, oneor both of the first and second optical devices 210, 220 may be anoptical port for coupling to an external chip or device. The OWI 230includes integrated waveguide polarizers 200, each of which configuredto suppress light propagating in the optical waveguide interconnect inan undersized polarization mode, for example TM0, so as to reducepolarization crosstalk downstream from the integrated waveguidepolarizers 200. In one embodiment the polarizers 200 may be insertedwithin OWI 230 at regular intervals so as to avoid long lengths ofrouting waveguides without a polarizer. Waveguide polarizers may be forexample in the form of the waveguide polarizer 100 described hereinabovewith reference to FIGS. 1-6 but may also be different waveguidepolarizers that are configured to strip light passing therethrough forman undesired mode or modes, such as for example TM0 mode andhigher-order TEn modes. Although five polarizers 200 are shown in FIG.15, it will be appreciated that in other embodiments fewer or greaternumber of polarizers may be used, in dependence upon the total length ofthe waveguide interconnect and PIC design considerations. Thus, in oneembodiment OWI 230 may be in the form of, or include, a chain of two ormore polarizers 200 directly connected to each other in series byrouting waveguides, such as waveguide sections 231, 232, 233, and 234illustrated in FIG. 15. Polarizers 200 that are disposed in the routingpath 238 of the OWI 250 next to the optical devices 210, 220 andindicated in the figure by dashed blocks serve to clean-up light theoptical signal from undesired mode or modes as it is being transmittedby the first optical device 210 or received by the second optical device220; these polarizers may be omitted in some embodiments depending onthe optical devices used and system design tolerances.

The placement of the polarizers within the PIC may be determined at thestage of the PIC design in accordance with a pre-defined rule or analgorithm, for example so as to ensure the absence of polarizer-freerouting links in the PIC that are longer than a pre-defined maximumlength Lmax. The exact value of Lmax may vary in dependence uponparticular PIC requirements, parameters of the routing waveguides,presence of other integrated optical devices in the link, etc. It may bedetermined, for example, so as to ensure that the mode and/orpolarization extinction ratio ER of an optical signal stays below apre-defined maximum value ERmax as the optical signal propagates in thePIC, so as to reduce signal noise due to mode and/or polarizationcross-coupling to a system-acceptable level. Such a rule or a set ofrules may be incorporated in PIC layout software so as to provide anautomatic placement of optical polarizers in a PIC at the stage of thePIC layout design.

Accordingly, an example method of designing a photonic integratedcircuit (PIC) chip may include the following steps: a) determining arouting path in the PIC chip for an optical waveguide interconnect thatis configured for routing optical signals between optical elements; andb) disposing one or more waveguide polarizers along the routing path sothat a maximum length of a contiguous section of the optical waveguideinterconnect without a waveguide polarizer does not exceed a predefinedmaximum length Lmax. In the example embodiment of FIG. 15, the ‘routingpath’ is designated by a dotted line 238 and is the path defined by thewaveguides 231-234.

In one embodiment the maximum routing waveguide length Lmax may bedetermined in dependence upon one or more waveguide parameters, such asfor example waveguide width. In one embodiment, steps a) and b) may beperformed automatically using a computer executing software instructionsfor implementing said steps. Techniques and approaches for incorporatingcorresponding polarizer placement rules as software instructions intoexisting or newly developed computer programs for PIC layout design willbe apparent to those skilled in the art on the basis of the presentdisclosure.

With reference to FIG. 16, a schematic plan view of an integratedoptical polarizer 200 is configured to strip input light 101 of anundesired second mode M2 while enabling light of a desired first mode M1to pass through the polarizer 200 substantially without loss or withonly a small loss in power. The polarizer 200 may be formed of, orinclude, a mode-selective expander (MSE) waveguide 210 of a variablewidth or widths that may be configured to selectively attenuate, orsqueeze out, the light that enters the polarizer 200 in the second modeM2 while enabling the light that enters the polarizer 200 in the firstmode M1 to propagate to an output 125. The MSE waveguide (MSE) 210, maybe in the form of a bi-level, or generally multi-level, rib orstrip-loaded waveguide that includes a core or ridge waveguiding region122 of a first width We that may vary in the direction of lightpropagation, as indicated by an arrow 131, and an outer lower slabwaveguiding region 123 adjoining the core ridge waveguiding region 122along either side thereof, the outer slab waveguiding region 123 beingof a second width Ws that may also vary in the direction of lightpropagation. The core ridge waveguiding region 122 is disposed toreceive light 101 of the first and second modes from an input opticalwaveguide 105. The core ridge and/or outer slab waveguiding regions 122,123 are configured to preferentially expand the light of the second modeM2, which may have a lower effective index n_(eff) than the first modeM1, from the core ridge waveguiding region 122 into the outer slabwaveguiding region 123, as schematically indicated by a curved arrow132, and to propagate the first mode M1 along the core ridge waveguidingregion 122 for coupling into an output waveguide 125, as schematicallyindicated at 131, so that the outer slab waveguiding region 123 remainssubstantially absent of the first mode. The MSE 210 may be disposed on aPIC chip 250 as a part of a PIC. The first mode M1 may be, for examplethe TE0 mode of the input waveguide 105, while the second mode M2 may bea TM0 mode or a higher-order TMn or TEn mode of the input waveguide 105having a lower effective index than the first mode. The outer slabwaveguiding region 123 is generally greater in width than the inputoptical waveguide 105, while the core ridge waveguiding region 122 ofthe MSE 110 may be smaller in width than the input optical waveguide 105along at least a portion of the length of the MSE 110.

The MSE waveguide 210 may include an input taper section 114 wherein theouter slab waveguiding region 123 gradually widens in the direction oflight propagation and the core ridge waveguiding region 122 graduallynarrows in the direction of light propagation. The input taper section114 is optically followed by a mode squeezing waveguide section 116wherein the core ridge waveguiding region 122 remains narrower than inthe input waveguide 105 along a waveguide length L_(sq), with asubstantially constant core width Wc1 that is smaller than the corewidth We in the input waveguide 105.

In the illustrated embodiment of FIGS. 16 and 17, the mode squeezingwaveguide 116 may be followed by an output filter section 218, which maybe configured to filter out unwanted light squeezed out in the modesqueezing waveguide section 116, but reflected back towards the outputwaveguide 125. The output filter section 218 may comprise filterelements 221 and 222 in the slab waveguiding region 123 on oppositesides of the output waveguide 125, respectively. Ideally, each filterelement 221 and 222 includes a wedge-shaped or triangular-shaped sectioneither cut out from or formed, e.g. doped, in the slab waveguidingregion 123 forming a first angled surface 223 and 224 at a first acuteangle, e.g. 10° to 60°, ideally 15° to 30°, from the output waveguide125. When the first acute angle is measured CCW from the outputwaveguide 125 the angle may be −10° to −60°, ideally −15° to −30°. Thefirst angled surfaces 223 and 224 are generally at the first acute angleto the sides or the longitudinal axis of the output ridge waveguide 125.The first angle surfaces 223 and 224 may include a light reflective orlight absorbing layer and/or the cut out sections may comprise amaterial, e.g. air or glass, including an index of refraction (e.g.η<1.5) different, e.g. lower, than the slab waveguiding region 123 (e.g.η>3.4) forming a refractive index contrast. The wedged-shaped sectionsmay also be filled, e.g. doped, with a material forming a lowerrefractive index contrast or a light reflective or absorbing material.Accordingly, the filter elements 221 and 222 reflect or absorb any straylight away from the output waveguide 125. The wedge-shaped sections 221and 222 may also include a diffraction grating that may be formed, forexample by etching therein or in the top cladding layer 143. Thediffraction grating may have a period selected to deflect the undesiredlight of the undesired second mode or modes away from the ridgewaveguide 122 and from the polarizer output 125. For example, thediffraction grating may be a second-order grating designed to deflectlight of the operating wavelength λ, out of the PCI chip 250 or backtowards an absorbent material, e.g. absorber 160.

To enhance coupling of the first mode into the output waveguide 105, anapex 225 and 226 of the cut-out section of each filter element 221 and222, respectively, is spaced from the output waveguide 125 by a thintapering section 227 and 229, respectively, of the slab waveguidingregion 123. The thin tapering section 227 and 229 tapers down from amaximum width, e.g. less than 2 μm, ideally between 0.5-1.0 μm,proximate the apexes 225 and 226 to zero, i.e. when the output waveguide125 becomes a ridge waveguide. Accordingly, a second angled surface 231and 232 is formed between the filter sections 221 and 222 and the slabwaveguiding region 123, at a second acute angle, e.g. 2°-10°, to theoutput waveguide 125 At the interface proximate the apexes 225 and 226,there is an abrupt transition from a strip-loaded waveguide, e.g.greater than 8 μm wide, ideally between 8 to 12 μm wide, to astrip-loaded waveguide that has reduced a width, e.g. 1.5-2.5 μm wide;however, the mode overlap integral between the two modes is 0.9999 forthe first mode and should not affect the propagation thereof. The apexes225 and 226 point in the opposite direction to the direction of lightpropagation 131, whereby the first angled surfaces 223 and 224 arerearwardly facing, i.e. partially facing towards the input taper section114 and the mode squeezing section 116.

FIG. 16 illustrates an example embodiment wherein the outer waveguidingregion 123 gradually widens along a first length portion 112 of theinput taper section 114, while the core waveguiding region 122 remainsat a constant width. Subsequently, the core waveguiding region 122gradually narrows in width while the outer slab waveguiding region 123remains at a constant width along a second length portion 113 of theinput taper section 114, which follows the first length portion 112 inthe direction of light propagation. In other embodiments the widening ofthe outer waveguiding section 123 may occur in parallel with thenarrowing of the core 122, along a same length portion of the polarizer200.

Referring back to FIGS. 2, 3, and 4 while continuing to refer to FIGS.16 and 17, in one embodiment the input waveguide 105 may be a ridgewaveguide of height h1 and width Wc, while the MSE 210 may be in theform of a bi-level strip-loaded waveguide, which may also be referred toas the bi-level rib waveguide 210, wherein the core waveguiding region122 in the form of a ridge or rib of height h1 is flanked on one or bothsides by a slab waveguide of height h2<h1 defining the outer waveguidingregion or regions 123. The outer slab waveguiding regions 123 may alsobe referred to herein as the slab waveguide 123. The first lengthportion 112 of the MSE 210, wherein the slab waveguide 123 widens whilethe width of the core ridge waveguide 122 may remain the same, may alsobe referred to in this embodiment as the ridge-to-rib waveguide taper112, while the second length portion 113 wherein the core ridgewaveguide 122 narrows may also be referred to as the rib waveguide taper113. FIG. 2 shows a cross-sectional view of the input ridge waveguide105 taken along the A-A line indicated in FIG. 16. FIGS. 3 and 4 showcross-sections of the bi-level strip-loaded waveguide 210 in the inputtaper section 114 and in the mode squeezing waveguide 116, which aretaken along the lines B-B and C-C indicated in FIG. 16, respectively.FIG.3 shows the cross-section of the ridge-to-rib waveguide taper 112,wherein the width Wc of the core 122 remains constant and matching thewidth of the input ridge waveguide 105, while the width Ws of the outerslab waveguiding portion 123 gradually widens. In the rib waveguidetaper 113 the width Wc of the waveguiding core 122 gradually decreasesto the smaller width Wc1 of the waveguide core in the squeezingwaveguide 116, which cross-section is illustrated in FIG. 4.

In other embodiments, the input waveguide 105 may also be a bi-level ribwaveguide, for example with an outer slab waveguide of a differentheight h3 h2, with the first length portion 112 of the input taper 114providing an adiabatic transition and mode matching between the inputwaveguide 105 and the rib waveguide taper 113.

Continuing to refer to FIGS. 1-4, the input ridge waveguide 105 and thebi-level rib waveguide 210 may be disposed in or upon the PIC chip 250and formed, for example, by selectively etching to a desired depth asemiconductor or dielectric layer 133 that is disposed over a base layer142. The base layer 142 has a lower index of refraction than thematerial of the core and outer waveguide regions 122, 123 of the ribwaveguide 210, and serves as a lower cladding layer of the ridge or ribwaveguide. The bi-level rib waveguide 210 may also be formed by atwo-step selective deposition process of same or different materialsthat forms top and bottom layers of different width. In one embodiment,a top cladding layer 143 may be deposited over the rib waveguide 210. Inanother embodiment, the top layer 143 may be absent, with the uppercladding provided by air or another gas. In one embodiment the PIC chip250 may be a silicon-on-insulator (SOI) chip 250, the layer 133 in whichthe core and outer waveguiding regions 122, 123 of the rib waveguide 210is formed may be a silicon layer of the SOI chip 250, and the base layer142 may be a layer of silicon dioxide (SiO2) disposed over a siliconsubstrate 141 of the SOI chip 250. The top cladding layer 143, whenpresent, may be for example a layer of silicon dioxide, silicon nitride(Si3N4), polymer, or of any other suitable non-conducting material of alower index of refraction than that of the waveguiding regions 122, 123.It will be appreciated that other material systems may also be used toform the bi-level rib waveguide 210.

In one embodiment, the first mode M1 may be the fundamental TE0 mode ofthe input ridge waveguide 105 having the highest effective index neffand therefore the largest optical confinement in the waveguide core,with the TM0 mode and the higher-order TE modes TEn having lowereffective indices than that of the TE0 mode. The narrowing down of thecore ridge waveguiding region 122 along the second length portion 113 ofthe input taper 114 squeezes the TM0 mode and the higher-order TEn modesout of the core ridge waveguide region 122 and into the outer slabwaveguiding regions 123 to a significantly greater extent than thehigher-effective index first mode TE0. The widths of the slab waveguideWs and of the core waveguide Wc1 in the mode squeezing section 116 maybe selected so that optical power of the first mode TE0 is confinedsubstantially within the waveguide core 122, while the TM0 and TEn modesmay have most of the mode power spread in the outer waveguiding regions123 where the first mode TE0 is substantially absent or negligiblysmall, for example, no more than a few percent or tenth of a percent inpower. Furthermore, the geometry of the input waveguide taper 114 may beconfigured so that light that enters the MSE 210 in a TM0 mode may be atleast partially coupled into a higher-order TEn mode, such as TE1 andTE3, within a length of the ridge-to-rib waveguide taper 112, and isthen squeezed out of the waveguide core 122 into the side region 123 asthe waveguide core 122 becomes narrower in the rib waveguide taper 113.The TM→TE mode conversion of this type may be effected in the bi-levelrib waveguide taper 210 at a specific width Ws=Wsc of the outer regions123 due to its “vertically asymmetry,” i.e. an asymmetry in thedirection normal to the surface of the PIC wafer or layer upon which thewaveguide 210 is formed, which is the z-axis direction in the examplecoordinate system illustrated in FIG. 16. The TM0→TEn conversion in theridge-to-rib taper 112 may facilitate the suppression of light thatenters the polarizer in the TM0 mode, for example, in embodimentswherein the effective index of the TM0 mode is higher than that of theTE1 mode in the narrow-core squeezing waveguide 116.

Generally, the geometry of the input waveguide taper 114, which may bedefined by the height parameters of the rib waveguide h1 and h2, thewidth Ws1 and Wc1 of the slab waveguide 123 and of the waveguide core122 at the output end of the input taper 114, respectively, may beselected so that light 101 that enters the MSE 110 in the first modehaving the greatest effective index in the input waveguide 105, such asfor example the TE0 mode, remains largely confined within the waveguidecore 122 at the output of the input taper section 114 and in the modesqueezing waveguide 116, while light 101 that enters the MSE 210 in thesecond mode having the lower effective index in the input waveguide 105,such as for example the TM0 mode or a TEn mode, largely loses itsconfinement within the waveguide core 122 by the time it reaches themode squeezing waveguide 116. It may be preferable that the input tapersection 114 is an adiabatic taper of a sufficiently long length, so thatthe change of its core width Wc and of the width of the outerwaveguiding sections Ws happens smoothly over a length that issufficient to prevent back reflections and to allow light that entersthe input taper section 114 in the second mode, for example TM0, tocouple into the higher-order modes and/or modes of the slab waveguide123.

Generally, the width and height parameters Wc, Wc1, Ws, h1, h2 of therib waveguide 210 of the polarizer 100 in various sections may depend onthe core and cladding materials of the waveguide and the targetwavelength range of operation, and one skilled in the art will be ableto determine suitable values using commercially available software forwaveguide simulations and experimental verification. By way of examplefor the polarizer 200 that is formed in a h1=220 nm thick silicon layerof a SOI chip, the input waveguide width Wc may be in the range of 0.4to 0.6 μm, the slab waveguide thickness h2 may be in the 50 to 160 nmrange, and the core waveguide width Wc1 of the squeezing waveguide 116may be down to 0.18 to 0.24 μm. The lengths of the ridge-to-rib taper112 L1 and of the rib waveguide taper 113 L2 may be selected to providean adiabatic transition between the input ridge waveguide 105 and thesqueezing rib waveguide 116; for example, each of L1 and L2 may be about10 μm or greater. The length of the squeezing rib waveguide 116 L3 maybe selected to provide a desired level of suppression of the unwanted TMand TEn modes and may also be for example about 10 μm or greater. Thewidth Ws of the outer regions of the rib waveguide in a middle portionof the structure 100 may be for example 5 μm or greater.

Note that although the narrowing of the waveguide core 122 in the ribwaveguide taper 113 and the widening of the slab waveguide 123 in theridge-to-rib taper 112 is shown in FIG. 16 to be linear with devicelength in the direction of light propagation, it is by example only andin other embodiments the narrowing of the core waveguide 122 and/or thewidening of the slab waveguide 123 may be non-linear with the distancealong the direction of light propagation, and the input taper section114 may be a multi-segmented and/or smooth taper, which exact shape inthe plane (x,y) of the chip 150 may be determined by optimization.

In one embodiment, light 132 of the unwanted second mode, such as theTM0 and/or TEn, that is squeezed out of the waveguide core 122 in thesqueezing waveguide 116, may be scattered away from the waveguide core122 so that only at most a small portion of it is coupled back into thewaveguide core by the outer taper 118; in some embodiments, suchscattering may be sufficient to provide a desired level of suppressionof the undesired modes at the output of the polarizer 200.

Referring to FIG. 17, in some embodiments the MSE 210 may include alight absorber/deflector 160 or 230 that is configured to selectivelyabsorb or deflect light propagating in the outer waveguiding region 123thereof, and in particular in the outer waveguiding regions of the modesqueezing waveguide 116, as hereinbefore defined with reference to FIGS.5 to 9.

The above-described exemplary embodiments are intended to beillustrative in all respects, rather than restrictive, of the presentinvention. Indeed, various other embodiments and modifications to thepresent disclosure, in addition to those described herein, will beapparent to those of ordinary skill in the art from the foregoingdescription and accompanying drawings. Thus, such other embodiments andmodifications are intended to fall within the scope of the presentdisclosure. For example, it will be appreciated that semiconductormaterials other than silicon, including but not limited to compoundsemiconductor materials of groups commonly referred to as A3B5 and A2B4,such as GaAs, InP, and their alloys and compounds, may be used tofabricate the integrated waveguide polarizers example embodiments ofwhich are described hereinabove. In another example, although exampleembodiments described hereinabove may have been described primarily withreference to polarizers or mode filters that are configured to block afundamental TM mode while letting a fundamental TE mode to pass through,it will be appreciated that principles and device configurationsdescribed hereinabove with reference to specific examples may be adoptedto squeeze out and eliminate any waveguide mode or group of modes of alower effective index than a desired waveguide mode that is to be passedthrough by the polarizer, by suitably configuring the waveguide geometryof the MSE waveguide 110 or 210. For example, embodiments may beenvisioned wherein a TM0 mode enters the polarizer with a greatereffective index than the TE0 mode, for example depending on the geometryof the input waveguide, in which case the first, or desired, mode M1 maybe the TM0 mode, the second, or undesired, mode M2 may be the TE0 modeor a higher-order TM or TE mode. Furthermore, although in the exampleembodiments described hereinabove the MSE waveguide 110 is shown to besubstantially straight, in other embodiments it may include one or morewaveguide bends, for example but not exclusively in the mode squeezingwaveguide section 116, which may further facilitate leaking of theundesired mode light out of the waveguide core and into the slabwaveguide, wherein it can be attenuated or diverted away from the outputwaveguide. Furthermore although in the example embodiments describedherein the MSE waveguide 110 or 210 is in the form of a bi-levelstrip-loaded or rib waveguide, multi-level implementations thereof mayalso be envisioned, and are within the scope of the present disclosure.

Although the theoretical description given herein is thought to becorrect, the operation of the devices described and claimed herein doesnot depend upon the accuracy or validity of the theoretical description.That is, later theoretical developments that may explain the observedresults on a basis different from the theory presented herein will notdetract from the inventions described herein.

Any patent, patent application, patent application publication, journalarticle, book, published paper, or other publicly available materialidentified in the specification is hereby incorporated by referenceherein in its entirety. Any material, or portion thereof, that is saidto be incorporated by reference herein, but which conflicts withexisting definitions, statements, or other disclosure materialexplicitly set forth herein is only incorporated to the extent that noconflict arises between that incorporated material and the presentdisclosure material. In the event of a conflict, the conflict is to beresolved in favor of the present disclosure as the preferred disclosure.

While the present invention has been particularly shown and describedwith reference to the preferred mode as illustrated in the drawing, itwill be understood by one skilled in the art that various changes indetail may be affected therein without departing from the spirit andscope of the invention as defined by the claims.

What is claimed is:
 1. A waveguide polarizer, comprising: an inputoptical waveguide; an output optical waveguide; and a mode-selectiveexpander (MSE), extending between the input and output opticalwaveguides, comprising a bi-level rib waveguide comprising: a ridgewaveguiding region disposed to receive light from the input opticalwaveguide, and including a taper section in which the ridge waveguidingregion gradually decreases in width; and an outer slab waveguidingregion disposed alongside the ridge waveguiding region in opticalcommunication therewith, and including an input taper section in whichthe outer slab waveguiding region gradually widens in a direction oflight propagation.
 2. The waveguide polarizer according to claim 1,wherein the outer slab waveguiding region gradually widens along a firstlength of the input taper section, while the ridge waveguiding regionremains at a first constant ridge width, and wherein the ridgewaveguiding region gradually narrows along a second length of the inputtaper section following the first length in the direction of lightpropagation, while the outer slab waveguiding region remains at aconstant slab width.
 3. The waveguide polarizer according to claim 2,wherein the MSE also comprises a mode squeezing section in which theridge waveguiding region remains at a second constant ridge width lessthan a width of the input waveguide.
 4. The waveguide polarizeraccording to claim 1, wherein the ridge waveguiding region is disposedto receive the light including first and second modes from the inputoptical waveguide, and wherein the ridge waveguiding region and theouter slab waveguiding region are configured to expand the light of thesecond mode from the ridge waveguiding region into the outer slabwaveguiding region, and to propagate the first mode along the ridgewaveguiding region for coupling into the output optical waveguide, sothat the outer slab waveguiding region remains substantially absent ofthe first mode.
 5. The waveguide polarizer according to claim 4, furthercomprising an output filter section for preventing the second mode fromentering the output optical waveguide.
 6. The waveguide polarizeraccording to claim 5, wherein the output filter section comprises a pairof angled surfaces, each at an acute angle to the output waveguide onopposite sides thereof, both facing substantially rearwardly towards theMSE for reflecting light in the slab waveguiding region away from theoutput waveguide.
 7. The waveguide polarizer according to claim 6,wherein the acute angle is between 10° and 60°.
 8. The waveguidepolarizer according to claim 6, wherein the output optical filtercomprises a pair of wedge-shaped sections in the outer slab waveguidingregion on opposite sides of the output waveguide forming the angledsurfaces; and wherein the wedge-shaped sections, each comprises amaterial with a lower index of refraction than the slab waveguidingregion.
 9. The waveguide polarizer according to claim 8, wherein eachwedge-shaped section includes an apex spaced from the output opticalwaveguide pointing rearwardly towards the MSE.
 10. The waveguidepolarizer according to claim 9, wherein each wedge-shaped section isspace from the output waveguide by a tapering section of the outer slabwaveguiding region.
 11. The waveguide polarizer according to claim 8,wherein each wedge-shaped section further comprises: an absorbent dopinglayer.
 12. The waveguide polarizer according to claim 8, wherein eachwedge-shaped section further comprises a reflective layer for reflectingthe light in the outer slab waveguiding region away from the outputwaveguide.
 13. The waveguide polarizer according to claim 8, whereineach wedge-shaped section further comprises a grating surface forscattering the light in the outer slab waveguiding region away from theoutput waveguide.
 14. The waveguide polarizer according to claim 1,further comprising an optical shielding disposed about the MSE toprevent light in the outer slab waveguiding region to couple into anexternal optical device and/or to shield the MSE from external light.15. The waveguide polarizer according to claim 1, further comprising alight absorber configured to selectively absorb light propagating in theouter slab waveguiding region.
 16. The waveguide polarizer according toclaim 5, wherein the light absorber comprises at least one of: a metallayer coupled to the outer slab waveguiding region, a layer oflight-absorbing semiconductor material coupled to the outer slabwaveguiding region, and a doped region of a semiconductor layer formingthe outer slab waveguiding region.
 17. The waveguide polarizer accordingto claim 1, further comprising a light redirecting element configured toselectively re-direct light propagating in the outer slab waveguidingregion thereof away from the output optical waveguide.
 18. The waveguidepolarizer according to claim 7, wherein the light redirecting elementcomprises one of: an optical grating formed in the outer slabwaveguiding region or optically coupled therewith, and an auxiliarywaveguiding layer optically coupled to the outer slab waveguiding regionto re-direct light propagating therein away from the ridge waveguidingregion.
 19. The waveguide polarizer according to claim 4, wherein thefirst mode is characterized by a greater effective refractive index inthe MSE than the second mode.
 20. The waveguide polarizer according toclaim 4, wherein the first mode is a fundamental TE mode, and the secondmode is a TM mode or a higher order TE mode.